The mechanisms regulating hematopoietic stem cell (HSC) proliferation, self-renewal, and differentiation are fundamental to our understanding and treatment of a number of hematopoietic disorders, including bone marrow failure syndromes and hematopoietic malignancies. Accumulating evidences suggests that stromal cells in the bone marrow provide key signals regulating HSC, yet the nature of these signals and even the stromal cell types that comprise the stem cell niche are poorly understood. Recent studies from our laboratory show that treatment with granulocyte colony-stimulating factor (G-CSF) results in marked changes in the bone marrow microenvironment that culminate in the mobilization of HSC into the blood. In this proposal, we plan to continue this research to answer the two following questions: 1) what are the cellular components of the stem cell niche; 2) what are the signals that regulate the number and function of stem cell niche cells. This research has direct clinical relevance for stem cell transplantation, since it may lead to novel strategies to increase HSC mobilization yields and enhance HSC homing and engraftment following transplantation. Preliminary data support the hypothesis that factor(s) produced by cells of the monocyte lineage in the bone marrow provide key signals that regulate osteoblast function/survival and that expression of these factor(s) is regulated by G-CSF. In Aim 1, we will identify candidate factors produced by monocytic cells and characterize their contribution to G-CSF-induced HSC mobilization and osteoblast suppression. The identification of factors that regulate osteoblasts may lead to the development of small molecule mimetics (or antagonists) that are able to stimulate (or inhibit) osteoblast (and possibly HSC) function. The chemokine Cxcl12 provides a key signal regulating HSC quiescence, survival, and trafficking. Within the bone marrow, Cxcl12 is expressed in osteoblasts, osteoblast precursors, endothelial cells, and Cxcl12- abundant reticular (CAR) cells. In Aim 2, we will selectively delete Cxcl12 in selected stromal cel types and characterize their effect on HSC function and trafficking. To this end, we have generated transgenic mice carrying a floxed null allele of Cxcl12. These studies should provide new insight into the cellular composition of the stem cell niche and may lead to better strategies to augment HSC function in bone marrow failure syndromes or after stem cell transplantation. The following specific aims are proposed. Aim 1. We will define the contribution of bone marrow monocytes/macrophages to HSC mobilization by G- CSF. Aim 2. We will define the contribution of Cxcl12 expression by osteoblasts and endothelial cells to HSC function and trafficking.